Re: st: RE: Interpretation of quadratic terms

The behavior is a reflection of the problem of collinearity. When
"age" is used in the model, the collinearity is not so severe and
"margins" is able to produce an estimate. But, when "year" is used in
the model, the collinearity is so severe that "margins" is not even able
to produce an estimate. That was the point of the example, that
sometimes collinearity is more of a problem than other times, and that
sometimes centering is necessary and other times centering is not necessary.

Dear Eric,
I apologise for my lack of clarity regarding the -margins- command. Even though I have hardly used it, exchanges on the Statalist have made it clear that -margins- can do many things, including the calculation of marginal effects. However Michael did not seem to have used the correct syntax; I believe that the option "dydx" must be used to obtain marginal effects.
Rodolphe
________________________________________
From: owner-statalist@hsphsun2.harvard.edu [owner-statalist@hsphsun2.harvard.edu] On Behalf Of DE SOUZA Eric [eric.de_souza@coleurope.eu]
Sent: 11 March 2010 15:01
To: 'statalist@hsphsun2.harvard.edu'
Subject: RE: st: RE: Interpretation of quadratic terms
Dear Rodolphe,
I don't have Stata 11 either. But according to the preface to the new edition of Cameron and Trivedi, Microeconometrics using Stata:
"Second, we describe the new margins command for prediction and for computation of
marginal effects in regression models. The margins command with options including the
dydx() option replaces the Stata mfx command and the user-written margeff command.
Additionally, the margins command when used in conjunction with factor variables can
simplify computation of marginal effects in models with interactions. "
So the margins command is Stata 11 does allow one to calculate marginal effects
Eric de Souza
College of Europe
BE-8000 Brugge (Bruges)
Belgium
-----Original Message-----
From: owner-statalist@hsphsun2.harvard.edu [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Rodolphe Desbordes
Sent: 11 March 2010 14:59
To: statalist@hsphsun2.harvard.edu
Subject: RE: st: RE: Interpretation of quadratic terms
I do not have Stata 11 installed on this computer and I am not vary familiar with the -margins- command. However, using -margins-, you did not calculate the marginal effect but the predicted value of wage, at a given value of age.
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